In general, as an exhaust purification apparatus located in an exhaust system for an internal combustion engine such as a diesel engine, an NOx catalyst is known which is used to purify NOx (nitrogen oxide) contained in exhaust gas. Various types of NOx catalysts are known. Among these NOx catalysts, an NOx catalyst of selective reduction type is well known to which a reducing agent is added to continuously reduce and remove NOx. As a reducing agent, urea is commonly used in the form of a water solution. A urea aqueous solution is injected and fed into exhaust gas present on the upstream side of the catalyst. Then, the urea aqueous solution receives heat from the exhaust and the catalyst and is thus hydrolyzed to generate ammonia. The ammonia reacts with NOx on the NOx catalyst, causing the NOx to be decomposed into N2 and H2O.
In an exhaust purification system using the NOx catalyst of selective reduction type, if the urea aqueous solution fails to be appropriately added to the NOx catalyst, exhaust emission may be degraded. Thus, desirably, the urea addition system is appropriately diagnosed for abnormalities. If an abnormality is detected, the user is immediately notified of the abnormality to take appropriate measures such as repair. In particular, in the field of automobiles, regulations in many countries require that abnormalities that may degrade the exhaust gas be detected onboard so that the driver can be warned of the abnormalities.
In connection with conventional abnormality diagnoses for urea addition systems, for example, Japanese Patent Application Laid-Open No. 2005-83223 manages the concentration of a urea aqueous solution by providing a concentration detection device that detects the concentration of the urea aqueous solution, in a tank in which the urea aqueous solution is stored.
Abnormalities in the urea addition system which may degrade the exhaust emission include an addition amount abnormality in which the urea addition amount deviates from a preset value and a quality abnormality in which the properties (particularly the concentration) of the urea aqueous solution deviate from preset ones. The former may occur when an abnormality occurs in the urea addition device itself, for example, a urea injection valve becomes defective. The latter may occur when, for example, a urea aqueous solution other than a specified one or plain water is refilled into a urea aqueous solution tank. In any case, in terms of prevention of degradation of the exhaust emission, both abnormalities can desirably be detected. Furthermore, when at least one of the abnormalities is detected, the user is desirably warned of the abnormality. In this connection, the technique described in Japanese Patent Application Laid-Open No. 2005-83223 can detect the abnormality in the concentration of the urea aqueous solution but not the addition amount abnormality.
Furthermore, in connection with the abnormality in the quality of the urea aqueous solution, such a urea aqueous solution concentration detection device as described in Japanese Patent Application Laid-Open No. 2005-83223 is generally expensive and thus unsuitable for being mounted in a vehicle. Furthermore, the provision of the urea aqueous solution concentration detection device means the addition of the new device. This complicates the configuration of the apparatus and increases costs.